A radar system may measure the distance to a target through measurement of the round-trip time-of-flight of the radar signal to a target and return. The one-way distance, d, to a target is computed from the equation 2d=ct where c is the velocity of light and where t is the time between transmitted signals and received signals reflected from a target. Radar technology is well developed. However, an individual target may be difficult to isolate if there are many reflectors in the field of the radar system. Also, the distance calculation may be corrupted by multipath effects and limited bandwidth of the transmitted pulse. Multipath effects may be mitigated by using the time of the first return signal from a target. However, this technique is fraught with problems if the reflected signal from the desired target is hidden by signals reflected from other objects in the field of the radar system.
RFID systems are well known in the art and are used to monitor objects and places by attaching tags to objects and places to be monitored. These objects may be large and in the presence of many other reflecting objects. RF signals from a tag may be hidden by noise and larger signals reflected from other reflecting objects. Backscatter RFID readers transmit CW signals while acquiring data from tags, and thus lack the capability of ranging using measurement of time-of-flight methods. RFID tags used in modulated backscatter RFID systems are often referred to as ‘passive’ (without an internal source of power) or ‘semi-passive’ (with an internal source of power) since modulated backscatter tags do not generate radio signals and only reflect radio signals. RFID tags may also send data to a RFID reader by generating and sending radio signals. These types of RFID tags are often referred to as ‘active’ tags since they generate radio signals and contain an internal source of power. The phase of the backscattered signals from a modulated backscatter tag can be used to calculate the distance to a tag in the presence of other reflecting objects, as disclosed in U.S. patent application Ser. No. 12/840,587, titled SYSTEM AND METHOD FOR MEASUREMENT OF DISTANCE TO A TAG BY A MODULATED BACKSCATTER RFID READER, but accuracy may be degraded in a highly reflecting environment due to multipath effects. Many types of RFID systems use modulation signals with frequencies on the order of a megahertz or less and often shape waveforms to comply with radio regulations. These modulation waveforms lack nanosecond precision needed to use time-of-flight methods to measure distance to a resolution on the order of a meter or less between tags and readers.
RFID systems using time-of-flight methods to determine object location may be found in the art but these types of systems are expensive, require careful installation, use expensive tags and require precise positioning of the system components. Signal strength methods to determine tag location may also be found in the art, but these types of systems lack accuracy and precision.
Many tens of millions of RFD tags are presently in use and installations would benefit if the distance to these tags could be measured accurately in a complex radio environment.